Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
FIG. 1 depicts a prior art ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards an end station 120.
The end station 120 supports one or more workpieces, such as workpiece 122, in the path of ion beam 10 such that ions of the desired species are implanted into the workpiece 122. The workpiece 122 may be, for example, a semiconductor wafer or other similar target object requiring ion implantation. The end station 120 may also include a platen 124 to support the workpiece 122. The platen 124 may secure the workpiece 122 using electrostatic forces or other similar forces. The end station 120 may also include a scanner (not illustrated) for moving the workpiece 122 in a desired direction. The end station 120 may also include additional components, such as automated workpiece handling elements for introducing a workpiece 122 into the ion implanter system 100 and for removing the workpiece 122 after ion implantation. The ion implanter system 100 may also include a controller (not illustrated) to control a variety of subsystems and components of the ion implanter system 100. The ion implanter system 100 may also include a number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 128, and a setup Faraday cup 126. These devices may be used to monitor and control the ion beam conditions. It should be appreciated by those skilled in the art that the entire path traversed by the ion beam 10 is evacuated during ion implantation.
The ion source 102 is a critical component of the ion implanter system 100. The ion source 102 is required to generate a stable, well-defined ion beam 10 for a variety of different ion species and extraction voltages. It is therefore desirable to operate the ion source 102 for extended periods of time without the need for maintenance or repair. Hence, the lifetime of the ion source 102 or mean time between failures (MTBF) is one performance criteria of the ion source 102.
FIG. 2 depicts a typical embodiment of the ion source 102 in the ion implanter system 100. The ion source 102 may be an inductively heated cathode (IHC) ion source, which is typically used in high current ion implantation equipment. Other various ion sources may also be utilized. The ion source 102 includes an arc chamber housing 202 defining an arc chamber 206. The arc chamber housing 202 also includes an extraction aperture 204 for the ion beam 10. A cathode 208 and a repeller electrode 210 (or anticathode) may be positioned within the arc chamber 206. The repeller electrode 210 may be electrically isolated. A cathode insulator 212 may be positioned relative to the cathode 208 to electrically and thermally insulate the cathode 208 from the arc chamber housing 202. The cathode 208 may also be separated from the insulator 212 by a vacuum gap to control thermal conduction. A filament 214 may be positioned outside the arc chamber 206 and in close proximity to the cathode 208 to heat the cathode 208. A support rod 216 may support the cathode 208 and the filament 214. One or more source magnets 220 may also be provided to produce a magnetic field B within the arc chamber 206 in a direction toward the cathode 208 (see arrow 222 of FIG. 2).
An extraction electrode configuration, such as a ground electrode 240 and a suppression electrode 242, may be positioned in front of the extraction aperture 204. Each of the ground electrode 240 and the suppression electrode 242 have an aperture aligned with the extraction aperture 204 for extraction of the well-defined ion beam 10 from the arc chamber 206 for use in the ion implanter system 100.
An extraction power supply 248 may provide an extraction voltage to the ground electrode 240 for extraction of the ion beam 10 from the arc chamber 206. The extraction voltage may be adjusted according to the desired energy of the ion beam 10. A suppression power supply 246 may bias the suppression electrode 242 to inhibit movement of electrons within the ion beam 10. One or more additional power supplies may also be provided to the ion implanter system 100, such as a filament power supply or an arc power supply. A filament power supply (not illustrated) may provide current to the filament 214 for heating thereof, which in turn generates electrons that are accelerated toward the cathode 208 for heating the cathode 208. An arc power supply (not illustrated) may be coupled to the arc chamber housing 202 to accelerate electrons emitted by the cathode 208 into a plasma 20 formed within the arc chamber 206.
An ion source controller 250 provides control of the ion source 102. For example, the ion source controller 250 may control various power supplies of the ion source and/or may also control the flow rate of dopant gas from a dopant gas source 260 into the arc chamber 206. The ion source controller 250 may be a programmed controller or a dedicated special purpose controller. In one embodiment, the ion source controller 250 is incorporated into a main control computer of the ion implanter system 100.
A dopant gas source 260 may inject a predetermined amount of dopant gas into the arc chamber 206 via a gas flow controller 266. The dopant gas source 260 may provide a particular dopant gas containing a desired dopant element. For example, the dopant element may include boron (B), germanium (Ge), phosphorus (P), or silicon (Si) and may be provided as a fluorine-containing gas, such as boron trifluoride (BF3), germanium tetrafluoride (GeF4), phosphorous trifluoride (PF3), or silicon tetrafluoride (SiF4). Other various dopant gases and/or dopant elements may also be utilized, such as inert gases, including argon (Ar), xenon (Xe), etc.
A common cause of ion source failure is that some materials accumulate on cathode surfaces during extended ion implantation processes. The accumulated materials tend to reduce a thermionic emission rate of source ions from cathode surfaces. Consequently, desired arc currents cannot be obtained and the ion source 102 may have to be replaced in order to maintain normal ion source operation. As a result, performance degradation and short lifetime of the ion source 102 greatly reduces the productivity of the ion implanter system 100.
The above-described problems are especially significant for, but are not limited to, germanium ion implantation. Germanium ion implantation has been widely used in the semiconductor industry to pre-amorphize silicon wafers in order to prevent channeling effects. The demand for pre-amorphizing germanium ion implantation is expected to increase greatly in future semiconductor device manufacturing. One of the most popular source gases for germanium ion beams is germanium tetrafluoride (GeF4) due to its stable chemical properties and cost-effectiveness. However, very short lifetimes of ion sources have been observed while operating with GeF4 dopant gas.
The short lifetime of an ion source used in germanium ion implantation may be attributed to the presence of excessive, free fluorine atoms in the arc chamber 206 as a result of chemical dissociation of GeF4 molecules. Specifically, arc chamber housing 202 material may be etched away due to chemical reactions with these free fluorine atoms. The arc chamber housing 202 material may eventually be deposited on a surface of the cathode 208, resulting in the degradation of electron emissions from the surface of the cathode 208.
It should be appreciated that while problems with germanium ion implantation are discussed above, other fluorine-containing dopant gases, such as boron trifluoride (BF3), phosphorous trifluoride (PF3), and silicon tetrafluoride (SiF4), may exhibit similar problems that adversely affect performance and lifetime of the ion source 102 as a result of such materials deposited on the cathode 208. Although an inert gas, such as argon, xenon, etc., may be used as a dopant gas, using inert gases, even though they do not contain fluorine, inevitably result in reduced beam currents. As a result, ion source performance and lifetime is still greatly reduced.
Another common cause of ion source failure is caused by stripping (or sputtering) of cathode material. For example, metallic material (e.g., tungsten (W), molybdenum (Mo), etc.) from the cathode 208 is inclined to react with ions from plasma 20 in the arc chamber 206 accelerating towards the cathode 208. Because sputtering is dominated by the heaviest ion in the plasma 20, as ion mass increases, the sputtering effect may worsen. In fact, continued sputtering of material “thins” the cathode 208 and may eventually lead to an aperture or opening within the cathode 208. Consequently, performance and lifetime of the ion source 102 are greatly reduced when utilizing a dopant gas containing a heavy element, such as germanium (Ge), arsenic (As), xenon (Xe), etc., as opposed to lighter elements, such as boron (B) or carbon (C). These adverse effects are particularly noticeable when using hydrides (e.g., AsH3, PH3, CH4, etc.), inert gases (Ar, Xe, etc.), or a mixture thereof, as the source of desired implantation species.
In view of the foregoing, it would be desirable to provide a technique for improving the performance and extending the lifetime of an ion source to overcome the above-described inadequacies and shortcomings.